Fix pkgdown: add IMPACT_files to reference index

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bschilder

_pkgdown.yml

@@ -125,3 +125,7 @@ reference:
       - DEEPLEARNING_query_one_chr
       - DEEPLEARNING_melt
       - DEEPLEARNING_plot
+
+  - title: Bundled data
+    contents:
+      - IMPACT_files